Prior to the illumination of some gas discharge lamps, a small current is passed through the lamps to warm the filaments. This filament preheating allows the strike voltage for the lamp to be lowered, thereby allowing the lamp to light almost immediately when the strike voltage is applied.
In order to apply the optimum amount of current to warm the filaments, the resistance of the filament is often measured. The resistance of the filament prior to preheating is referred to as “rcold.” The resistance of the filament after it has been warmed is referred to as “rhot.” The ratio of rhot to rcold is often referred to as Rh/Rc. Knowing the approximate value of rhot and rcold for lamps allows ballasts to be designed specifically for the lamps.
Measurement of the rhot and rcold for a lamp is a complex process. In order to ascertain the rhot for a lamp, a technician measures the resistance of the lamp filament with an ohm meter. To obtain rhot, the lamp is attached to an oscilloscope. A current is applied to each lamp filament. After a short length of time, the voltage across the filament is then measured. From the measurements of current and voltage, the resistance of the lamp filament is obtained.
While this method of measuring rhot and rcold is effective, it has shortcomings. First, the measurement process is very time consuming. Thus, only a few lamps are tested for rhot and rcold. Second, the process measures rhot only after the lamp filament has been fully heated. Therefore, possible problems with the heating of the lamp filament prior to the time it is fully heated are not revealed.
An improved method of determining rhot and rcold which is less time consuming and provides more information about the relationship between rhot and rcold is thus highly desirable.